Membrane separation techniques have been hitherto used for seawater desalination, water purification treatment, gas separation, hemocatharsis and the like, in which researches have been recently advanced to apply the membrane separation techniques to waste water treatment from the viewpoint of environmental protection.
Hitherto, sand filtration, gravity sedimentation and the like have been carried out as processes for accomplishing solid-liquid separation of treated water high in turbidity, such as water purification treatment, sewage and waste water treatment, treatment of industrial waste water and the like. However, the solid-liquid separation by these processes have problems such as occurrence of a case where the water quality of obtained treated water becomes insufficient or of requiring a vast site for the solid-liquid separation.
As processes for solving these problems, a variety of studies have been made in recent years on a process for accomplishing the solid-liquid separation of treated water by using a membrane module provided with a separation membrane such as a precision filtration membrane, an ultrafiltration membrane or the like. With these processes, the filtration treatment for treated water is carried out by using a separation membrane, and therefore treated water high in water quality can be obtained (see, for example, Non-patent Citation 1).
In case of accomplishing the solid-liquid separation of treated water by using a separation membrane, clogging at a separation membrane surface with suspended matter proceeds with continuation of filtration treatment, thereby resulting in lowering in filtering flow rate or rising of a differential pressure across the membrane. In order to restore such a condition, as shown in FIG. 8, an air diffusing device 2 is disposed below a membrane module 23 to accomplish a process for peeling off the suspended matter at the separation membrane surface by shaking (scrubbing) the treated water at the surface of a separation membrane 5 upon making diffusion of air from the air diffusing device 2 (see, for example, Patent Citations 1 and 2). As a supply source for scrubbing air to the air diffusing device 2, a blower and a compressor is used.
In the membrane separation apparatus disclosed in Patent Citations 1 and 2, one air diffusing device 2 (diffuser pipe) is disposed for each separation membrane 5 in order to cause air bubbles 6 for scrubbing to act uniformly and sufficiently on whole the separation membrane 5. Further, in order to improve the dissolving efficiency of scrubbing air into treated water, the diameter of air bubbles is made small.